Io is the innermost of the Galilean moons of Jupiter. It is the most volcanically active object in the solar system and the most dense of all planetary satellites. Io was named for an Argive princess and priestess of Hera who became a fairly prominent mistress of Zeus, after much suffering. That name, assigned quite by accident in the seventeenth century, might be quite apt today in view of Io's volcanic activity and the likely reason for it.

Discovery and naming

Galileo Galilei observed Jupiter beginning on January 7, 1610, with his famous telescope. He at first thought he had discovered three stars near Jupiter, but on the next night those "stars" seemed to have moved. (Those "stars" were actually Io, Europa, and Callisto.) He continued to observe Jupiter and its companion "stars" for seven days, during which time a fourth "star" (Ganymede) appeared and all four of these objects appeared to move with Jupiter. Finally he concluded that these objects were not stars at all, but satellites of Jupiter. This was the first direct observation that provided evidence for Nicolaus Copernicus's heliocentric model of the solar system.[1]

Simon Marius claimed to observe Jupiter and these satellites independently of Galileo and beginning five weeks earlier. However, he did not publish his findings, while Galileo published his. Furthermore, Galileo's notes were more reliable and extensive than those of Marius, which is why Ganymede and the three other satellites he observed (Io, Europa, and Callisto) are called the Galilean satellites and not the Galilean-Marian satellites.[1]

Marius does, however, receive credit for providing the names that the satellites have today. He named Ganymede, the largest, for the mythical son of King Tros of Troy, carried aloft to Mount Olympus by Zeus to be the cupbearer to the gods. (The names Zeus and Jupiter refer to the same classical deity from whom Jupiter gets its name.) The other three satellites are named for three of Zeus' most famous mistresses.[1] Marius propounded these names after Johannes Kepler suggested them to him.

Galileo, for his part, called them the "Medicean planets" after the Medici family and simply numbered the moons I, II, III, and IV. The satellites carried these names for two centuries until the discovery of other moons of Jupiter made that naming system untenably confusing.

Orbital and rotational characteristics

Orbital resonance of Io, Europa, and Ganymede

Io is fifth in order of all of Jupiter's moons, and orbits Jupiter at a semi-major axis of 421,800 km. Its orbit is very slightly eccentric and slightly inclined to Jupiter's equator. Io is in tidal lock (thus keeping the same face toward Jupiter as it revolves).

Io maintains a three-part orbital resonance or Laplace resonance with Europa and Ganymede. These three satellites complete their orbits around Jupiter in the ratio 1:2:4. They also conjoin with one another at their apsides, in a mutually self-correcting manner that allows the resonance to persist. (Most such resonances are unstable and end with one or all bodies changing their orbits.)

The origin of the resonance itself is controversial. Peale and Lee, in 2002, presented a model suggesting that this resonance is primordial and thus was part of the Jovian system since its formation.[7] Their model, however, derives from the nebular hypothesis of the formation of the solar system and thus depends on uniformitarian assumptions. Showman and Malhotra suggest a model by which the resonance developed after the solar system had formed.[8]

This orbital resonance, together with tidal stresses imposed by Jupiter itself, is widely held to be the most likely cause of Io's volcanism (see below).

Physical characteristics

Io cutaway

Io is slightly more massive than is Earth's Moon, though smaller in radius. These two factors, a consequence of Io's greater density, give Io a slightly higher surface gravity than the Moon has.

Io is much less heavily cratered than are the other three Galilean satellites. In uniformitarian terms, therefore, Io is considered "young," in contrast to the much "older" Ganymede.[9]

Interior

Io is more dense even than Earth's Moon and in fact is more dense than any other satellite of Jupiter (or any other planet).[5] For that reason, and taking into account mass-concentration and other findings from flybys of all the Galilean satellites, Io is not suspected of having an underground saltwater ocean. Instead, it likely has a metal core of iron and/or iron sulfide. According to the best model available so far, the radius of this core likely extends to half the total radius of Io itself. Surrounding this would be a partially molten silicate-rich mantle with a thin rocky surface crust.[4][10]

Further evidence that Io has an iron core comes from magnetometric measurements taken by Galileo. The magnetic flux density at Io's equator is about 13 milligauss.[6] This corresponds to a magnetic dipole moment of 7.86 * 1019 N-m/T. Considering Io's mass, Io likely had a magnetic dipole moment at creation of 2.1 * 1022 N-m/T, according to Russell Humphreys' model for the creation of celestial magnetic fields.[11] This in turn suggests a decay time of 1098 Julian years and a half-life of 761 Julian years. The corresponding decay time and half-life of the magnetic field of Ganymede agrees with this to within four significant digits, and suggests that both satellites have comparable core radii and conductivities.

The marked difference between Io's interior and those of its three outer counterparts has not gone unremarked. Uniformitarian thinkers suggest that Jupiter was at its hottest early in its initial accretion, and that lighter elements and compounds would not have been able to accrete as close to Jupiter as Io's orbit was. In this scenario, Jupiter is held to be the center of a "mini-solar system" with heavier elements and compounds accreting inward and lighter substances accreting outward.[4][12]

Volcanism

Mercator-like projection of Io showing active volcanoes

Io's most striking physical characteristic is its tremendous volcanic activity. The volcanoes of Io, which Voyager 1 helped discover in 1979, were the first extraterrestrial volcanoes ever discovered.[13]

Io has hundreds of active volcanoes, and their eruptions and lava flows are constantly changing the surface of Io. The active eruption sites achieve temperatures as high as 1800 K, this although the average surface temperature of Io is about 130 K.[4]

The favored theory for explaining this volcanism is tidal heating. The orbital resonance that Io has with Ganymede and especially Europa, combined with the eccentricity of Io's own orbit around Jupiter, combine to flex Io's crust by as much as 100 meters between Io's perizene and apozene.[9][14] The resonance, furthermore, helps maintain the eccentricity and also prevent Io from receding from Jupiter.

Sodium cloud released by volcano Prometheus on Io

The tremendous observed flexing during a sidereal month of less than two Julian days would be expected to heat the mantle tremendously. In fact, Io is known to radiate heat at a power level of about 125 TW on average. This yields a heat flow per surface area of 2.5 W/m².[14]

Yet the tremendous heat of Io's volcanoes is not fully understood or explained. If uniformitarian theories are correct and Io is millions of years old, the innumerable tidal-flexing cycles should have lowered Io's melting point so that it would not produce such hot lava flows. The high temperatures observed are in fact consistent with basaltic lava flows that most uniformitarians think occurred billions of years ago on Earth. No volcano on Earth today is as hot as are some of the volcanoes now on Io.[15]

Mountains

Io's mountains average 6 km in altitude, and some of Io's mountains are as high as 16 km, higher even than Mount Everest on Earth.[16] Most of these are not volcanoes, because they have no associated lava flow. Most scientists believe that these mountains form by faulting.

Yet volcano-related depressions, or calderas, often surround some of Io's peaks.[16] The locations of many of the calderas violates many accepted assumptions concerning volcanism and mountain formation.[17] Debate on the origins of these calderas continues today, with some observers favoring fractures in Io's crust and others favoring the more traditional explanation of the collapse of vacated magma chambers.[17]

Atmosphere

Io has no atmosphere in the traditional sense, despite the tremendous amounts of sulfur dioxide and other gases that Io's many volcanoes release. Io does retain trace amounts of sulfur dioxide at a pressure of one micro-atmosphere or less. This is thick enough to generate auroras as the radiation from the Jovian radiation belt strikes the gas, but is not thick enough to have any refractive properties that would be detectable with stellar occultation.

The most likely explanations are Io's deep cold and Jupiter's magnetic field. Most of the released sulfur dioxide falls back to the surface as frost. The rest is stripped away as Io moves through Jupiter's magnetic field and radiation belt.[16]

Problems for Uniformitarianism Posed by Io

Io is radiating more heat than tidal heating could generate. Therefore, Io must have another source of heat, or else the observed heat flow is unusually high and not something that Io could maintain for long.[9] And even in the latter case, stating that the current heat flow is unusual begs the question of what drove the heat flow to present levels.

As mentioned before, the volcanic eruptions on Io have one-third the absolute temperature at the surface of the Sun. This defies explanation, because according to all current uniformitarian theories, Io's mantle ought to melt at far lower temperatures after millions of years of constant reworking by tidal flexes.[15]

Observation and Exploration

Until the advent of space flight, observations of Io were perforce limited to those by Earth-based telescopes. Galileo Galilei had shown that Io was present, of course, but only in the late 19th and 20th centuries did telescope technology allow for any resolution of surface features on Io. Astronomers like Edward E. Bernard noted the differing brightness between Io's equatorial and polar regions and finally determined that they were strictly due to differences in color and albedo (a measure of surface reflectivity), and not because Io had an odd shape and not because Io was in fact two companion objects.[18][19][20] More detailed observations followed, as recently (to the Voyager missions) as 1973.[21]

The Pioneer missions 10 and 11 flew by Io in 1973 and 1974. They revealed little beyond a better estimate of Io's mass, and one good image from Pioneer 11. The heavy radiation in the torus of radioactive particles that surrounds Io compromised Pioneer's instruments, and all its observations were lost.

The Galileo Project spacecraft did not make any close passes at Io for years, primarily because mission controllers were concerned with the safety of the spacecraft and desired to examine several far less dangerous targets. The mission was extended twice, however, and in the last years of the mission, Galileo made several flybys of Io and took surface temperature measurements. These measurements clearly showed that Io's volcanoes tended to be hotter even than many volcanoes on Earth.

In September 2003, the Galileo controllers deliberately crashed the spacecraft on Jupiter, in order to avoid crashing it on Europa and possibly doing irreparable damage to Europa's subsurface ocean. Since that time, observations have been limited largely to Earth-based telescopes, like the Keck Observatory in Hawaii and the Hubble Space Telescope. The New Horizons mission also made a flyby of Io on February 28, 2007, and captured many remarkable images. No other missions to Io are planned for at least ten more years.

↑The concept of Jupiter as a proto-star that somehow failed to ignite is fairly widely held. This model states that Jupiter, composed as it is of hydrogen and helium, would have ignited had it been sufficiently massive. But in that event, life on Earth could never have developed. See Harmsworth, Andrew, "Entry for Jupiter," Spacetech's Orrery: the Solar Sytem in Action, last update July 2, 2007. Accessed February 18, 2008.